Long Term Evolution (LTE) is the latest standard under development for mobile network technology. It is designed to enable wireless providers using both GSM and 3G networks to transition to fourth generation (4G) networks and equipment. For consumers, LTE will enable existing applications to run faster, and will also make available new mobile phone applications. In order to obtain the higher data rates required for these new applications, LTE has adopted multiple-input multiple-output (MIMO) technology, which will require mobile phones to have two cellular radio antennas. LTE also uses lower frequencies than the GSM band and mobile phone antennas will now have to have low band performance extended down to 698 MHz (from 824 MHz at present). This combination of needing two antennas and lower frequency performance presents significant problems for the designer of antennas for mobile platforms.
In order for a pair of antennas to give good diversity performance or work successfully in a MIMO system they need to sample, to a certain extent, different multipath signals arriving at the equipment terminal. This means, in effect, that the antennas must be different in some way by having different beam patterns, different polarisations, phase responses or be physically well separated electrically (spatial diversity).
An indication of how similar two antennas are is given by the envelope correlation coefficient ρe, which is a measure of how the radiation patterns of two antennas differ in shape, polarization and phase. A low correlation is very important for the performance of a MIMO system because when ρe=1 the patterns are identical and no MIMO or diversity gain is possible. However, when ρe=0 optimal MIMO gain is achieved. It is important to note that that the overall performance of the two antennas must be similar; good MIMO performance cannot be achieved using one efficient antenna and one inefficient antenna. Both must have similar efficiencies, but be different in one or more of the characteristics listed above.
Recently it has been shown that loop antenna technology can be used for mobile phone applications and, by means of switching or electronic tuning, can be configured to cover the LTE bands as well as the GSM bands, for example as described in the present Applicant's co-pending UK patent application no GB0914280.3. Recent developments designed to improve bandwidth include multi-moding the loops, complex feed and grounding arrangements and complex structural arrangements towards the centre of the loop designed to improve the match to 50 ohms. Some of these developments are described in detail in the present Applicant's co-pending UK patent application no GB1017472.0, the content of which is incorporated into the present application by reference.